A digital, solid-state camera comprises imaging optics for imaging a scene onto an image sensor. The image sensor comprises a one- or two-dimensional array of picture elements (pixels) that transduce the incident light into electric signals; it is typically a solid-state device using the charge-coupled device (CCD) or the complementary-metal-oxide-semiconductor (CMOS) technology. The electric signals are read out and processed.
For capturing color images, each pixel of the image sensor has to selectively be sensitive only to certain light wavelengths. The wavelength (or color) selectivity is usually achieved by a color filter on top of each pixel. Filters for different colors are distributed in certain patterns on the pixel array. The most common pattern of filters is the Bayer filter pattern which alternates a row of red and green filters with a row of blue and green filters. Seldom, other patterns and/or colors are used.
The filters themselves are usually executed as absorption filters. Such an absorption filter is a layer of organic material comprising an organic dye that essentially transmits one color—e.g., red—and absorbs all other colors—e.g., green and blue. The main drawback of absorption filters is their limited lifetime. The organic materials degrade and age. The aging process is speeded up by external influences such as ultraviolet (UV) radiation, high temperature, humidity, etc. Consequently, the quality of a conventional digital camera decreases with time. Further disadvantages of image sensors with absorption filters are the complexity and the expensiveness of their manufacture. In order to manufacture the filters for one color, a corresponding layer is deposited on the chip, a mask defining the pixels for detecting the corresponding color has to be provided, the layer is exposed through the mask, and the superfluous areas of the layer are removed. This procedure has to be performed for each of the at least three colors—e.g., the primary colors red, green and blue. Moreover, the spectral characteristics of the absorption filters are often not precisely defined and difficult to adapt to a desired behavior.
Diffractive zero-order filters are known in optics, e.g., from U.S. Pat. No. 4,484,797. They consist of non-absorbing diffractive structures, i.e., phase objects, made of different material layers with different refractive indices. Their typical dimensions (grating periods) are equal to or smaller than the light wavelength. These diffractive filters are mainly used as optically variable security devices in banknotes, credit cards, passports etc. To date, diffractive optics has hardly been used in microelectronics. The technologies are believed to be incompatible, and the dimensions of the diffractive structures are often in the same range or larger than those of the microelectronic device and thus not applicable to it.